That's a question for the FCC. Here is a chart showing the US radio frequency allocation. General answer is that prior to the late 1980's the U.S. only had 11 freqs in the 72-75 MHz band. 5 (in 72 MHz)were shared use for any model; the remaining 6 (the only 1 on 75 MHz) were aircraft only. The Academy of Model Aeronautics took the lead for the whole model hobby industry and petitioned the FCC for the new "1991" freqs, with 50 channels for aircraft only on 72 MHz; and 30 channels for surface models on 75 MHz. The new narrowband freqs were space 20 KHz apart, the older freqs were at least 40 KHz apart. The older wide band radio gear became illegal (by FCC rules) to use in 1998. There were still people using wideband stuff in 2005. I was at a regatta that year and couldn't run my model because another guy was on channel 70 and glitching out channels 69 and 71.